It is proposed to study the development of the vertebrate nervous system, primarily in rats. The temporal gradients of neuroblast origin, assembly, and differentation, including axon growth, will be analyzed comprehensively along the radial and longitudinal axes of the embryonic nervous system. It is proposed to determine if the same pattern of origin and assembly of neuroblasts occurs along the entire rostrocaudal axis. It is also proposed to determine if the temporal sequence of axon growth along the orientated interfaces in the radial and longitudinal axes leads to the topographic organization of the fiber tracts in the central nervous system. This proposal will involve three levels of organization: 1. To determine in the brain stem of rats, the pattern of neuroblast origin and assembly as revealed by (3H) thymidine autoradiography, and the pattern of axon growth as revealed by silver stains. 2. To adapt to chick and rat embryos methods of analyzing axonal pathways and terminals, i.e. Nauta silver stains and E.M. following lesions, and (3H) leucine autoradiography of axonally transported protein. 3. To test in the ascending fiber tracts in the embryonic spinal cord, the predicted temporal sequence of axon growth with the methods in "2". This research should provide understandings to the development of the cytoarchitecture and interconnections, and to the evolution of the nervous system. It should provide some basic information for studying birth defects, teratogenic agents, embryonic behavior, and also other aspects of axon growth patterns, i.e. biochemical specificity, synaptogenesis, etc.